A semiconductor device manufacturing process includes various steps such as a lithography step, a dry etching step, and an ion implantation step. After each step is finished, the following processes are carried out before the operation moves on to the next step: a cleaning process to remove impurities and residues remaining on the wafer surface and clean the wafer surface; a rinsing process to remove the chemical solution residues after the cleaning; and a drying process.
For example, in the wafer cleaning process after the etching step, a chemical solution for the cleaning process is supplied to the wafer surface. Pure water is then supplied, and the rinsing process is performed. After the rinsing process, the pure water remaining on the wafer surface is removed, and the drying process is performed to dry the wafer.
As the methods of performing the drying process, the following methods have been known: a rotary drying method by which pure water remaining on a wafer is discharged by utilizing the centrifugal force generated by rotations; and an IPA drying method by which pure water on a wafer is replaced with isopropyl alcohol (IPA), and the IPA is evaporated to dry the wafer. By those conventional drying methods, however, fine patterns formed on a wafer are brought into contact with one another at the time of drying due to the surface tension of the liquid remaining on the wafer, and as a result, a blocked state might be caused.
To solve such a problem, supercritical drying to reduce the surface tension to zero has been suggested. In the supercritical drying, after the wafer cleaning process, the liquid on the wafer is replaced with a solvent such as IPA to be replaced with a supercritical drying solvent at last. The wafer having its surface wetted with IPA is guided into a supercritical chamber. After that, carbon dioxide in a supercritical state (a supercritical CO2 fluid) is supplied into the chamber, and the IPA is replaced with the supercritical CO2 fluid. The IPA on the wafer is gradually dissolved in the supercritical CO2 fluid, and is discharged together with the supercritical CO2 fluid from the wafer. After all the IPA is discharged, the pressure in the chamber is lowered, and the supercritical CO2 fluid is phase-changed to gaseous CO2. The wafer drying is then ended.
By another known method, a supercritical CO2 fluid is not necessarily used as the drying solvent, and alcohol such as IPA serving as a substitution liquid for the rinse pure water after the cleaning with the chemical solution is put into a supercritical state. The alcohol is then evaporated and discharged, to perform drying. This technique is readily used, because alcohol is advantageously liquid at ordinary temperature and has a lower critical pressure than that of CO2. At high pressure and temperature, however, the alcohol has a decomposition reaction, and the etchant generated through the decomposition reaction performs etching on the metal material existing on the semiconductor substrate. As a result, the electrical characteristics of the semiconductor device are degraded.